The invention relates to a magnetic sensor and a method of producing the same and, particularly, relates to a magnetic sensor having a ferromagnetic tunnel junction used for a read head for high density magnetic record or a sensor for sensing a magnetic field and a method of producing the same.
In a junction of a laminate structure in which a metal layer, an insulation layer, and a metal layer are laminated in this sequence (in this description, such a laminate structure is denoted as xe2x80x9cmetal/insulator/metalxe2x80x9d), it is known that in the case where the thickness of the insulation layer is substantially small (of the order of several hundreds picometers to several thousands picometers), a small electric current is passed when a voltage is applied between the metal layers on both sides. This phenomenon is called xe2x80x9ctunnel effectxe2x80x9d, and can be explained quantum-mechanically. Also, the current is called a tunnel current, and such a junction is called a tunnel junction.
As an insulation layer in such a tunnel junction, an oxidized metal film is conventionally used. For instance, a film of aluminum oxide formed by oxidizing a surface layer of aluminum by natural oxidation, plasma oxidation, or thermal oxidation is used as an insulation layer. The aluminum oxide film can have a thickness of the order of several hundreds picometers to several thousands picometers, which is needed for a tunnel effect, by control of the oxidation conditions.
A junction having a structure of ferromagnetic metal/insulator/ferromagnetic metal, which is formed to have metal layers of ferromagnetic material on both sides of a tunnel junction, is called a ferromagnetic tunnel junction. In this case, it is known that a-magnitude of tunnel current depends on the magnetization conditions of both ferromagnetic metal layers. The largest tunnel current is passed when the directions of magnetization of both layers are oriented in the same direction, and a small tunnel current is passed when the directions of magnetization of both layers are opposite to each other. It is explained that conductive electrons are polarized in a ferromagnetic material, and that this phenomena is caused by the electrons tunneling while retaining the polarization. The electron polarizing in one direction can only tunnel to the state in which electrons are polarized in that direction, and the electron polarizing in the opposite direction can, only tunnel to the state in which electrons are polarized in the opposed direction. When ferromagnetic substances of both metal layers sandwiching an insulation layer have the same direction of magnetization, tunneling can occur between the same states, and a tunnel current is large (a tunnel probability is high). When they have opposite directions of magnetization, tunneling cannot occur unless an electron in a state in which the electron polarizes in one direction and an electron in a state in which the electron polarizes in the opposite direction respectively find vacancies in states in the layer to which they are to tunnel and, in general, a tunnel current is small (a tunnel probability is small).
Thus, in the ferromagnetic tunnel junction, a tunnel probability (tunnel resistance) depends on the magnetization states of magnetic layers in both sides and, on this account, the tunnel resistance can be controlled by applying an external magnetic field to change the magnetization states of the magnetic layers.
In other words, a change in external magnetic field can be detected by the change in tunnel resistance.
A tunnel resistance R can be represented by the following equation:
R=Rs+(xc2xd)xcex94R(1xe2x88x92cos xcex8)xe2x80x83xe2x80x83(1)
wherein xcex8 is a relative angle of magnetization between both magnetic layers, Rs is a tunnel resistance when the relative angle xcex8 is 0xc2x0, i.e. the directions of magnetization of both magnetic layers are parallel, and xcex94R is a difference between a tunnel resistance in the case where the directions of magnetization of both magnetic layers are parallel and a tunnel resistance in the case where the directions of magnetization of both layers are anti-parallel.
As is understood from equation (1), a tunnel resistance is smallest when the directions of magnetization of both magnetic layers are parallel, and is largest when they are anti-parallel. This is caused by that electrons in a ferromagnetic substance being polarized in spin. An electron is commonly in either an upward-spinning state or a downward-spinning state. The electron in the upward-spinning state is called an up-spin electron, and the electron in the downward-spinning state is called a down-spin electron.
In a non-magnetic substance, the numbers of up-spin electrons and down-spin electrons are equal to each other. On this account, a non-magnetic substance does not show magnetic properties, as a whole. On the other hand, in a ferromagnetic substance, the numbers of up-spin electrons and down-spin electrons are different from each other. Accordingly, the ferromagnetic substance has an upward or downward magnetization, as a whole.
It is known that when electrons tunnel in a tunnel junction, respective electrons tunnel retaining their spin state. An electron can tunnel to a magnetic layer if the layer has a vacancy in energy level corresponding to the spin state of the tunneling electron, and cannot tunnel if there is no vacancy in energy level.
A rate of change in tunnel resistance, xcex94R/Rs, is represented by the following equation using a product of a polarizability of a magnetic layer of electron source and a polarizability of vacant energy level in a magnetic layer to which electrons are to tunnel:
xcex94R/Rs=2P1P2/(1xe2x88x92P1P2)xe2x80x83xe2x80x83(2)
wherein P1 denotes of a spin polarizability of electron of an electron source, and P2 denotes a spin polarizability of vacant energy level in a magnetic layer to which electrons are to tunnel. Further, P1 and P2 are represented as follows:
P1, P2=2(Nupxe2x88x92Ndown)/(Nup+Ndown)xe2x80x83xe2x80x83(3)
wherein Nup denotes the number of up-spin electrons or the number of levels for up-spin electrons, and Ndown denotes the number of down-spin electrons or the number of levels for down-spin electrons.
The polarizabilities P1, P2 depend the type of ferromagnetic material, and some materials may show a polarizability close to 50%. In this case, a rate of change in resistance of the order of several tens of percent can be expected, which is larger than a rate of change in resistance obtained by anisotropic magnetoresistance effect (AMR) or giant magnetoresistance effect (GMR). For example, it is theoretically predicted that a rate of change in resistance having a value of the order of 20 to 50% can be obtained when a ferromagnetic metal, such as Co, Fe, and Ni, is used in a magnetic layer, and values close thereto have been obtained experimentally. Thus, since a rate of change in resistance in tunnel effect is larger compared to that in conventional anisotropic magnetoresistance effect or giant magnetoresistance effect, an element using a ferromagnetic tunnel junction is expected to be applied to a magnetic sensor in the next generation of devices.
In a tunnel conjunction element, defects, such as pinholes, tend to be generated when a tunnel insulation film sandwiched between two magnetic layers has a small thickness. If a tunnel insulation film has an increased thickness in order to prevent the generation of pinholes, however, there is a problem of reduced rate of change in magnetic resistance.
On the other hand, when a ferromagnetic tunnel conjunction element is used as a magnetic sensor, in general, a magnetic field is applied while passing a constant current (sense current), and change in value of resistance is detected and is converted to a voltage to be output. The ferromagnetic tunnel effect is known to have dependency on applied voltage, and its rate of change in resistance varies dependent on the applied voltage. In FIG. 1, a representative result of measurements of dependency on applied voltage of the rate of change in resistance in ferromagnetic tunnel effect is shown. As is evident from FIG. 1, although the ferromagnetic tunnel junction element has a larger rate of change in resistance at a small voltage, the rate of change in resistance is reduced to about half when a voltage of about 0.4 V is applied. It is thought that the voltage dependency in the ferromagnetic tunnel effect is caused by magnon (fluctuation of magnetic moment) occurring at the interface between a ferromagnetic substance and an insulator.
Although a larger output is obtained, in general, when a larger voltage is applied to an element, a ferromagnetic tunnel junction element provides, in fact, a small output because of the dependency on applied voltage when a large voltage is applied thereto. To solve this, a method in which bias properties are improved by connecting a plurality of tunnel junctions in series to thereby disperse an applied voltage to the respective elements, is disclosed (JP-A-11-112054). According to this method, however, since the junctions are connected in series, the total resistance value is increased.
As a property proper to the ferromagnetic tunnel junction, a non-linear voltage-current (V-I) characteristic is referred to. In FIG. 2, a representative V-I characteristic of the ferromagnetic tunnel junction is shown. Corresponding to this, a value of electric resistance (tunnel resistance) also greatly varies depending on an voltage, and shows a voltage-resistance (V-R) characteristic as seen in FIG. 3. From this, it is understood that, in a ferromagnetic tunnel junction, a resistance value has a great dependency on voltage. Consequently, there is a possibility that the great dependency of resistance value on voltage becomes a restriction on circuit design.
In view of these problems, it is an object of the invention to provide a magnetic sensor having a small reduction in a rate of change in magnetic resistance even at a large thickness of tunnel insulation film, and a method of producing it.
Also, it is another object of the invention to provide a ferromagnetic tunnel junction element having reduced or restricted dependencies of resistance value and rate of change in resistance on voltage, and a magnetic sensor using it.
According to an aspect of the invention, there is provided a magnetic sensor which comprises:
(1) a supporting substrate,
(2) a ferromagnetic tunnel junction element which has a first magnetic layer on the supporting substrate, a tunnel insulation layer on the first magnetic layer, the tunnel insulation layer comprising aluminum oxide obtained by oxidizing an aluminum film formed on the first magnetic layer by a sputtering using an aluminum target having a purity of 99.999% or more, and a second magnetic layer on the tunnel insulation layer, and
(3) a converter element converting a change in magnetic field to a change in resistance.
According to another aspect of the invention, there is provided a method of producing a magnetic sensor comprising (1) a supporting substrate, (2) a ferromagnetic tunnel junction element having a first magnetic layer on the supporting substrate, a tunnel insulation layer on the first magnetic layer, and a second magnetic layer on the tunnel insulation layer, and
(3) a converter element converting a change in magnetic field to a change in resistance, wherein the ferromagnetic tunnel junction element is fabricated by a process including the steps of: forming the first magnetic layer on the supporting substrate, sputtering an aluminum target having a purity of 99.999% or more to form an aluminum film on the first magnetic layer, oxidizing the aluminum film to convert it into the tunnel insulation layer comprising aluminum oxide, and forming the second magnetic layer on the tunnel insulation layer.
Using an aluminum target having a purity of 99.999% or more for the formation of aluminum film before the oxidation, a tunnel insulation layer obtained by the oxidation of the aluminum film can have a relatively large MR ratio even if the aluminum film formed has a large thickness. The initially formed aluminum film thus having a large thickness can be prevented from the generation of defects such as pinholes in this film, and can enhance the reliability of an insulation layer obtained by the oxidation of the aluminum film, to thereby enhance the reliability of a tunnel junction element comprising the insulation layer and also the reliability of an eventually yielded magnetic sensor.
Preferably, the pressure of an atmosphere before the sputtering of the aluminum target is not greater than 2xc3x9710xe2x88x924 Pa.
Preferably, the aluminum film on the first magnetic layer is oxidized in oxygen plasma to form a tunnel insulation layer comprising aluminum oxide.
According to a further aspect, there is provided a ferromagnetic tunnel junction element comprising a tunnel junction of laminated structure of ferromagnetic material/insulator/ferromagnetic material, wherein the tunnel junction has a voltage-resistance characteristic which is asymmetric in the direction of applied voltage. In this way, by providing a ferromagnetic tunnel junction, in which both resistance value and magnitude of change in resistance have a dependency on voltage, with modified dependencies of resistance value and magnitude of change in resistance on applied voltage which are different between the positive side and the negative side of the applied voltage, the magnitude of change in resistance as well as the resistance value of the tunnel junction element can be simultaneously reduced.
Preferably, the junction having the above-mentioned characteristic can be produced by the use, in an insulator layer, which is a barrier layer in a tunnel junction, of a material having a composition distribution which is asymmetric in relation to the direction of voltage applied, or the use of different materials in the respective layers in contact with the insulator layer.
In addition, dependency of resistance value of a ferromagnetic tunnel junction element on voltage may be reduced by connecting, in the element, two or more junctions in series in such a manner that their changes in resistance for the increase in voltage are developed in opposite directions.
In this tunnel junction element, by providing a voltage-resistance characteristic (V-R characteristic) which is asymmetric in the direction of voltage applied, the magnitude of change in resistance (xcex94R) of the ferromagnetic tunnel junction element is reduced with the application of voltage, while the resistance value (R) is also greatly reduced with the application of voltage, and, accordingly the reduction in rate of change in resistance (xcex94R/R) of the element can be made small. In the case of an element in which such junctions are connected in series so as to make the directions of change in resistance for the increase in voltage opposite to each other, the changes in resistance of the respective junction when a voltage is applied cancel each other, and the change in resistance becomes small in the whole element.
In this way, according to the invention, large changes in tunnel resistance value and rate of change in resistance in a ferromagnetic tunnel junction element due to an applied voltage can be avoided.
According to the invention, there is also provided a method of producing a ferromagnetic tunnel junction element comprising a tunnel junction of laminated structure of ferromagnetic material/insulator/ferromagnetic material, the method comprising imparting, to the tunnel junction, voltage-resistance characteristic which is asymmetric in the direction of voltage applied thereto.
According to a still further aspect of the invention, there is provided a magnetic sensor comprising:
(a) a supporting substrate,
(b) a ferromagnetic tunnel junction element comprising a tunnel junction of a laminated structure of ferromagnetic material/insulator/ferromagnetic material, the tunnel junction having voltage-resistance characteristic which is asymmetric in the direction of voltage applied, and
(c) a converter element converting a change in magnetic field to a change in resistance.